DESCRIPTION Hypothesis: Genetically distinct isolates of Entamoeba histolytica exist within a defined geographic area and vary in invasiveness and organ tropism. Entamoeba histolytica is a protozoan parasite that causes amebic dysentery and liver abscess. Virtually nothing is known about how many genetically distinct forms of E. histolytica exist in a human population at a given time. It is also not known if there are geographic variations, differences in invasiveness (colonization vs. dysentery), or altered organ tropisms (liver vs. colon) of genetically distinct E. histolytica isolates. The degree of genetic diversity within E. histolytica has important implications for both applied and basic amebiasis. For example, understanding the role of acquired immunity in preventing reinfection with E. histolytica depends on first deciphering the genetic homogeneity of E. histolytica and the contribution of different genotypes of E. histolytica to the different manifestations of infection (from asymptomatic colonization to invasive colitis and liver abscess). At a very practical level, the design of diagnostic tests and vaccines will need to take into account the genetic diversity of the organism. Dr. Haque (the foreign collaborator) and Dr. Petri (the US principal investigator) have found preliminary evidence of an association of E. histolytica zymodeme XIV with more severe disease in urban Dhaka. The five year collaboration has also resulted in the development of fecal antigen detection tests for E. histolytica which are rapid, sensitive, simple to perform in large numbers, and specific for E. histolytica. The investigators now propose to collect and genetically analyze isolates of E. histolytica from asymptomatic children and children with diarrhea and liver abscess, a process that will be greatly facilitated by the epidemiologic data already collected in Dhaka, and by the antigen detection tests that they have produced. The genetic heterogeneity of the isolates will be tested by isozyme classification into zymodemes, analysis of DNA polymorphisms of two single copy genes, and by polymerase chain reaction ribotyping. Successful completion of these studies will yield the first information on the genetic diversity of E. histolytica within a defined human population, and the contribution of E. histolytica genetic diversity to disease.